Wire
A wire could hardly find a more hostile environment than aboard a boat. On-board wiring is salted and doused, shaken and whipped, crushed and abraded, exposed to sunlight, subjected to heat, and coated with petroleum. Neither lamp cord nor house wiring will long endure these conditions.
Choose your wire carefully. Never use solid wire on a boat. Wave-or motor-induced oscillations eventually fracture solid wire. Boat wiring must have the flexibility stranding provides. Boatbuilders save a few dollars using type 2 wire, but a boatowner should use only the most flexible wire, called type 3.
The wire must be copper, but even copper corrodes in the marine environment, and corrosion is the primary cause of electrical failures on a boat. Plating each strand of the wire with a thin coat of tin-called tinning--dramatically improves corrosion resistance. The additional cost of tinned wire is nominal, the benefits substantial. Under normal circumstances use only tinned wire.
Stranded single-conductor wire is called hook-up or primary wire. Since most after-construction wiring requires two wires, duplex wire is more convenient and provides the added safety of a second layer of insulation. The best choice for most 12-volt wiring projects is duplex safety wire, where the twin conductors are red (positive) and yellow (ground). Making the ground wire yellow rather than black reduces the likelihood of confusing a DC ground wire with an AC hot wire-also black.
Boat cable
In recent years wire designed for the marine environment has become widely available to boatowners. This wire is known as boat cable. Unfortunately the Underwriter's Laboratories standard that defines boat cable, UL 1426, is less stringent than commonly thought. Boat cable can be type 2, tinning is not a requirement, and the heat rating of the insulation can be quite low. When you select boat cable, type 3 is better, tinning is essential, and you want the highest heat rating-designated on the jacket as BC5W2 (105¡C in a dry environment, 75¡C wet). Tinned boat cable from a reputable supplier is your best choice for all wiring needs.
Size
As with water through hose, electricity flows more easily through larger wire. It is essential to size wire for the maximum current flow you expect it to carry. If the wire feeds a single item, the current requirements will be shown on a label or plate on the appliance, or in the accompanying manual. If the rating is in watts, divide it by 12 (assuming a 12-volt electrical system) to convert the rating to amps. When the wire is part of a circuit that supplies several appliances, the potential current through the wire is the sum of the current requirements of every appliance on the circuit. For example, if a circuit is comprised of six 25-watt cabin lamps, the wire will be carrying about 12 amps ([25 watts Ö 12 volts] x 6) when all the lights are on.